1. Field of the Invention
The present invention relates to a developing unit used in an electrophotographic image forming apparatus, and more particularly to a developing unit securing the seal between the wall of the developer tank and the rotation shaft passing through the wall of the developer tank.
2. Description of the Prior Art
Referring to FIG. 6, an example of a conventional developing unit used in an electrophotographic image forming apparatus will be described:
The developing unit has a developer tank 10 which contains developer comprising toner and carrier. Inside the developer tank 10, there are rotatably mounted a developing roller 83 for applying the developer to a photosensitive drum 82, a pair of roller stirrers 84, 85 for stirring the developer, an agitator 87 for agitating the toner supplied through a toner supply opening 86, and a magnetic roller 88 for transporting the developer. The toner supplied through the toner supply opening 86 is agitated by the agitator 87 and is then stirred by the roller stirrers 84, 85 for mixing into the developer contained in the developer tank 10. The developer stirred by the roller stirrer 85 is transported to the developing roller 83 which carries the developer on the circumferential surface thereof, the amount of the developer being limited by a doctor blade 91 so that a prescribed amount of developer is applied to the photosensitive drum 82. The developer scraped by the doctor blade 91 is circulated, by means of a baffle plate 92 and the magnetic roller 88, back to the roller stirrer 84 directly or by way of a baffle plate 89.
FIG. 7 shows another example of the developing unit. The developing unit shown comprises a developer tank 10 in which a developing roller 83 for applying the developer to a photosensitive drum 82 and a roller stirrer 84 for stirring the toner supplied by a toner supply roller 97 for mixture into the developer are mounted. The developer stirred by the roller stirrer 84 is distributed to the photosensitive drum 82 by means of the developing roller 83, the amount of the developer carried on the developing roller 83 being limited by a doctor blade 91 so that a prescribed amount of developer is applied to the photosensitive drum 82. The toner density in the developer carried on the developing roller 83 is detected by toner density sensors 95a, 95b.
In either developing unit, the developing roller 83 which carries the developer on the circumferential surface thereof has a roller member 83a, a developer transport member, mounted on a rotation shaft 83c, as shown in FIG. 8. Each end of the roller member 83a is closed with a flange member 83b. On the other hand, as shown in FIG. 9, each of the roller stirrers 84, 85 has a number of stirring vanes 84a mounted at an angle on a rotation shaft 20, the stirring vanes 84a serving as transport members for axially transporting the developer by stirring. The stirring vanes 84a at both ends are mounted at right angles to the rotation shaft 20.
As shown in FIG. 10, the roller stirrers 84, 85 are rotatably supported in a wall 11 of the developer tank 10. That is, the rotation shaft 20 of each of the roller stirrers 84, 85 is rotatably supported in the wall 11 of the developer tank 10 via a bushing 99 formed from sintered material. The bushing 99 rotatably supports the rotation shaft 20, while the gap between the rotation shaft 20 and the bushing 99 is sealed with a sealing member 98. The sealing member 98 comprises a body portion 98a mounted in contacting relationship to the rotation shaft 20 and a conically shaped press-contact portion 98b gradually increasing in diameter with the distance from the body portion 98a. The body portion 98a of the sealing member 98 is glued to the stirring vane 84a at each end of the roller stirrer 84 (or 85) in such a way that the base of the press-contact portion 98b is press-contacted against an end face of the bushing 99. On the end portion of the rotation shaft 20 extending outside the developer tank 10, there is mounted a pulley 21 around which a belt 22 is applied. Driven by a drive unit (not shown), the belt 22 moves in a circular motion to rotate the pulley 21, thereby causing the rotation shaft 20 to rotate.
In such a developing unit, when the rotation shaft 20 is rotated with a radial load P applied to the pulley 21 because of the vibrations or other motion of the driving unit, the rotation shaft 20 may become eccentric with respect to the bushing 99, thus accelerating the wearing thereof. When the rotation shaft 20 becomes eccentric with respect to the bushing 99, the sealing member 98 contacts the bushing 99 unevenly, possibly causing abnormal wear of the sealing member 98. If such wear occurs, adequate sealing cannot be provided between the rotation shaft 20 and the bushing 99, and there is a possibility of developer leakage through the gap between the rotation shaft 20 and the bushing 99. Furthermore, when the bushing bearing 99 is formed from sintered material, the bushing 99 is heated to a high temperature due to friction between the rotation shaft 20 and the bushing 99. In particular, if the rotation shaft 20 becomes eccentric with respect to the bushing 99, the heating by friction will be accelerated and the bushing 99 will be heated to a temperature as high as 100.degree. C. The sealing member 98 is usually made of rubber such as chloroprene rubber. Therefore, when the construction is such that the sealing member 98 directly contacts the bushing 99, the problem is that since the heat is transferred to the sealing member 98, the deterioration of the sealing member 98 is accelerated and the sealing performance thereof drops after a short time of use.